Controllable pitch propellers are used to a considerable extent on watercraft, particularly ships. Variation in the propeller pitch facilitates maneuvering and permits an optimum propeller blade setting for the selected speed and power of the propeller drive motor.
Some sort of the power system is necessary for setting the pitch of the propeller blades. A manual adjustment device can only be used in connection with very small controllable pitch propellers bacause, with larger propellers, it is simply not possible to manually alter the propeller pitch while the vessel is being driven because of the large forces involved. Known adjustment devices include a lever operable from the vessel which acts on an adjustment sleeve mounted on the outflow side of the propeller hub and on which is arranged a thrust bearing forming a transition to the rotary system, i.e. the rotating propeller blades.
In the propulsion system of a larger ship, hydraulic adjustment drive systems can be used in which a single hydraulic cylinder causes the adjustment of the propeller pitch. A pressure medium, usually hydraulic oil, is necessary for the adjustment and fixing of the propeller blades and is introduced into the hub by means of a supply line while the used pressure medium, is led back through a return line. In such a hydraulic circuit, the transition from the stationary to the rotary portion of the controllable pitch propeller constitutes a problem area at which leakage losses can occur. However, additional leakage losses can also occur in the hub and the leaking pressure medium cannot be collected and returned. Thus, the hydraulic oil passes out into the water and becomes a source of pollution.
A completely hydraulic adjustment device of this general type is reliable but is costly and can only be economically installed on large ships.
On smaller ships and particularly on motorboats, the controllable pitch propeller has only been used to a limited extent because of the aforementioned problems.
The advantage of the fixed propeller is its simple design and the small dimensions of its housing. In a well-known solution, the housing of a fixed propeller is additionally employed for drawing off exhaust gases from the drive motor or engine for the watercraft. If the fixed propeller is replaced by a controllable pitch propeller, there is the disadvantage, that the mechanism for adjusting the pitch of the propeller blade is situated in the housing and therefore prevents the provision of several open spaces in the propeller hub for drawing off exhaust gases.